There exist throughout the United States, areas which have never been properly surveyed or profiled due to their remoteness from population centers, the relative inaccessibility of the areas, and/or a lack of money. In other areas of the country, resurveying or reprofiling of previously mapped areas is desirable to increase the accuracy and precision of or to update a previous mapping.
In each of these situations, surveying or profiling by traditional ground methods, that is, a ground survey using standard surveying equipment, may be either impractical due to the profile to the terrain or overly expensive and time consuming. Thus, for example, the terrain may be so rugged that it is too difficult or too large to be surveyed economically by traditional techniques.
It is therefore desirable to provide alternate methods and apparatus to the traditional ground survey profiling methods and apparatus hitherto known. One suggestion, (Svetlinchny, U.S. Pat. No. 3,741,653, issued June 26, 1973), has been to survey terrain from the air using laser tracking and scanning devices, secured in an airplane. The airplane flies a predetermined path while the laser trackers scan the ground and provide data describing the ground profile and airplane position. The data is stored on board the airplane for later ground processing and in some instances, preliminary on board processing may be performed. In general, the system requires (1) a range and angle tracking altimeter system (a scanning laser system) to determine the distance between the airplane and a sequence of scanned ground locations, and (2) at least two distance measurement devices (laser tracking systems) which provide aircraft distance to continuously tracked cooperative targets on the ground. The distance measurement devices supply data from which the position of the airplane can be calculated by triangulation.
More specifically, in operation, the prior art systems, which may not provide aircraft position information in real time, employ a first laser tracker to acquire one of the cooperating ground targets, and, while distance information is being provided from that target, a second laser tracker scans, locates, and tracks a second ground target. The distance from the tracker to the second ground target is then determined. By triangulation techniques, according to the Svetlinchny patent, using the angle between the pointing directions of the tracking devices, the position of the aircraft can be determined. Simultaneously, ground terrain range (and angle) information is gathered by a laser altimeter to determine the terrain profile. The terrain profile is also calculated by triangulation. In these prior art systems, as the aircraft begins to move out of sight of one of the cooperating ground targets, the laser tracker which had been following that target begins to search for and locates the next cooperative reflecting target and thereafter data from the third acquired target is used in combination with data from the second acquired target to determine the location of the aircraft by triangulation. As noted above, the laser altimeter provides the data necessary to complete the system.
It has also been suggested by Svetlinchny in U.S. Pat. No. 3,741,653, that it would be possible to use only one tracking device and one scanning laser altimeter in the terrain profiling system described above. While the exact operation of this particular configuration is not clearly provided in the reference, apparently the laser altimeter is designed to perform the dual functions of scanning the ground for cooperative targets and simultaneously recording altitude information. The various data are sorted out in an airborne computer. In operation, then, the laser tracker provides position data representing the range between the aircraft and a sequence of acquired cooperative reflecting targets on the ground. As the first target moves out of sight of the aircraft, the laser tracker switches to the next acquired target, its location having been previously determined by the laser altimeter scanning device. After acquiring the next reflecting target, the laser tracker again provides range information for triangulation determinations. During the time when the laser tracker is switching from one cooperating reflecting target to another, and only then, the airplane position may apparently be provided by a stable inertial platform. Thus there is available data from one source or another (but not both) to an onboard computer, to compute aircraft position.
The prior art systems are thus able to provide the necessary aircraft position information so long as the aircraft is within sight of at least one reflecting cooperating target, and may provide the required information even for a short time if a cooperating target is not being tracked, by using an inertial platform as the data source. Such prior systems are however severely limited because of errors which exist and grow in the inertial data. These errors may result from drifts in the measuring equipment, mechanical alignment errors, etc. In particular, the characteristics of the short term errors in the data from an inertial platform are not well known and may, very quickly, on the order of seconds, grow so that the position information provided by the inertial platform is inaccurate. As a result, performance of the prior art systems is limited and the accurately surveyed ground cooperating targets should be provided relatively close together so that the aircraft is never out of sight of all ground targets. This may be a difficult requirement to fulfill however because of the time and money required to properly position and survey a large number of cooperating targets; however, the prior art really provides no practical alternatives, because, if the aircraft loses sight of all ground targets, for a sufficiently long time duration (say 10 seconds), the cumulative errors in the prior art tracking systems will exceed the accuracy expected in high precision maps (about 10 feet in the horizontal).
The prior art aerial surveying or profiling systems are also severely limited because they cannot operate in real time and must perform substantially all of the final map determinations on the ground. Because of the large amount of data which is typically collected during a surveying run, the ground data analysis thereby tends to take an undesirably long time and money.
it is therefore an object of this invention to provide an airborne surveying apparatus and method which is capable of maintaining a specified high positional accuracy during aerial profiling even though the airborne vehicle is out of sight of all surveyed cooperative targets for a sustained time duration. Other objects of the invention are to provide an apparatus and method which is simple to operate, which can quickly and accurately survey large areas of terrain, which is reliable, and which does not require the relatively close placement of cooperating reflecting ground targets.
Further objects of the invention are to provide a method and apparatus which generates and provides error correction signals to an inertial position determining system in order to provide more accurate position location data and to perform substantially all calculations in real time thereby significantly reducing data storage requirements and the costs of post-processing the data. Yet other objects of the invention are to provide an aerial surveying apparatus and method having the capability of providing vertical coordinate information accurate to within plus or minus one-half foot, horizontal coordinate information accurate to within plus or minus ten feet, and to provide the information (assuming that the errors can be characterized by a normal distribution) within these specified limits 95% of the time.